Photovoltaic roof tiles: Design and integration in buildings

Abstract

The integration of photovoltaics (PV) into building facades and roof structures can provide a significant contribution to electricity generation. A design for a PV roof tile is proposed which will enable seamless integration with standard tiles in a roof structure. The constraints imposed by this requirement are discussed along with aesthetic, commercial and regulatory issues. In order to develop a photovoltaic roof tile it is first necessary to understand current roofing practice and materials. In the U.K., roof tiles form the primary barrier to the elements. The tiles are laid on wooden battens which run along the roof and are attached by special clips or nails. The battens are in turn nailed onto the rafters of the roof structure. A secondary level of protection is provided by a sarking felt which is laid between the rafters and the battens. This material provides insulation, prevents dust and air entering the roofspace and provides an extra waterproof barrier. A cut through image of a standard roof is shown (Fig. 1).The degree to which rows of tiles must overlap is defined by U.K.

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Photovoltaic roof tiles : design and integration in buildings
A. S. Bahaj & P.A.B.James, University of Southampton
1
Abstract
The integration of photovoltaics (PV) into building facades and roof structures can provide a significant
contribution to electricity generation. A design for a PV roof tile is proposed which will enable seamless
integration with standard tiles in a roof structure. The constraints imposed by this requirement are
discussed along with aesthetic, commercial and regulatory issues.
Introduction
Building integrated photovoltaics has a vast potential market in developed countries. Both commercial
and residential buildings have large surface areas which are available for PV integration. In urban areas
where land space is at a premium, the harnessing of such large areas is especially attractive. Roofs in
particular provide an ideal site for photovoltaic electrical power generation. In general they represent
large, flat surfaces which are less prone to shading than walls and exhibit more favourable inclinations
for solar gain. A typical U.K. roof for example is pitched between 17.5 and 44 degrees. For optimum
recovery, solar panels should be inclined perpendicular to the sun's rays. Clearly, a fixed solar array,
such as a roof mounted structure cannot satisfy this criteria and so a tilt angle of near 45 degrees is
likely to represent a good compromise.
Several options are available to achieve the integration of PV into roofs. In its simplest form, standard
size modules can be mounted, i.e. "bolted on", to the top of the present roof structure. This crude
approach does not truly integrate the PV with the building and produces the aesthetically poor result of
a high value product being merely "tacked on" to the current structure. Moreover, it is widely recognised
that it is important to offset the costs of PV installations by utilising modules not only for electrical
generation but also for building cladding / roofing and as an architectural device.
As a first step to achieving this goal PV roofs have been installed using standard modules which are laid
side by side across a roof deck. A capping is placed between the modules to ensure a watertight seal.
This process however, requires a complex aluminum framing structure to be mounted onto a precisely
formed predeck. The savings which result from the dual use of the photovoltaics are negated by the
high cost of the framing and mounting structure. Although the finished roof, represents a major advance
aesthetically over earlier bolt on solutions, the stepped effect obtained with traditional roofs, made of
tiles is lost. The large size of the modules used (upwards of 50 x 100 cm) accentuates the inaccuracies
that are present in the rafter and batten layout of a household roof structure forcing the production of
the highly ordered predeck. The battens used for standard tiling could be applied to PV integration if
smaller modules were used. A progression would be the production of a photovoltaic roof tile, which
would be analogous with standard tiles.
In order to develop a photovoltaic roof tile it is first necessary to understand current roofing practice
and materials. In the U.K., roof tiles form the primary barrier to the elements. The tiles are laid on
wooden battens which run along the roof and are attached by special clips or nails. The battens are in
turn nailed onto the rafters of the roof structure. A secondary level of protection is provided by a
sarking felt which is laid between the rafters and the battens. This material provides insulation, prevents
dust and air entering the roofspace and provides an extra waterproof barrier. A cut through image of a
standard roof is shown (Fig. 1).The degree to which rows of tiles must overlap is defined by U.K.
1
Institute of Cryogenics, Southampton University, Highfield, Southampton SO9 5NH

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building regulations (minimum of 75 mm). The exposed length and width of the tile are known as the
gauge and cover width respectively. In the U.K. there are two main manufacturers of roof tiles, namely
Redland and Marley who produce not only a diverse range of tile sizes but also of styles (Fig. 2).
Several tiles have raised edges (Redland Delta and Renown) or are curved (Redland Regent and
Grovebury) and so are not suitable for PV integration due to the associated shading and the planar
nature of current photovoltaic cells.
Figure 1: Section of a standard UK roof
Figure 2: A selection of tile styles
In addition to the variability in tile size and style, the type of interlock used between tiles is tile specific.
Clearly, to design a tile which satisfies the requirement of matching all types is an impossible task. The
design of a variety of interlock connectors which could be mounted onto a template PV tile to produce
the required shape is a possible solution [1]. However, the authors believe that this approach would
raise the cost of the PV tile to a prohibitive level. Manufacturing a variety of linkages is detrimental to
economies of scale and such a design will lengthen installation times. A typical domestic roof is 6 m
deep and 8 m across. Approximately 500 standard tiles would be needed to cover this area at a tile
cost of about £250-00 (tiles excluding clips ~ 47 p each). If a photovoltaic roof tile is designed around a

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specific standard tile, the majority of retrofit applications will require the current roof tiles to be
discarded. The cost of discarding unsuitable tiles will represent a small fraction of the overall system
cost and so will not effect the commercial viability of retrofit applications for PV tiles.
PV roof tile design
Accepting the prerequisite of a planar tile for integration yields a variety of suitable tiles for the design
of an analogous PV tile. The PV tile must have the same depth and interlock as its standard counterpart
but may have a cover width that is an integer multiple of the standard tile (e.g. 2x, 3x, 4x). Table 1 lists
a range of suitable tiles and the resulting potential dimensions for a PV roof tile.
Table 1. Suitable roof tiles for PV integration
A standard roof has a lifetime in excess of 25
years. To successfully promote a photovoltaic roof
a comparable lifespan must be offered. At
present, photovoltaic technology is available in
three distinct types, thin film, amorphous and
monocrystalline. Monocrystalline technology is well
established and module lifetimes of 20 years plus
can confidently be expected. Both amorphous and
thin film technologies are the subject of intensive
research and potentially will provide significantly
cheaper cells and modules. However, both
technologies suffer from performance degradation
with use and lifetimes of only a few years can be
guaranteed. Monocrystalline therefore, represents
the current technology that a PV tile must embrace
to produce the required product. The dimensions
of monocrystalline cells produced by all
manufacturers are virtually identical being 100 -
104 mm square. An optimum size array of these cells
must be fitted into the exposed surface of the PV tile. Figure 3: Design of a tile with integral PV cells
Table 1 shows that a Redland Stonewold II tile for
example, of double the normal cover width
provides an exposed area of 355 mm by 686 mm.
A 5 by 3 array of PV cells occupies an area of 379
mm by 579 mm which neatly fits into the exposed
area, whilst retaining a sufficiently large surround
for structural integrity. The proposed design is for
a laminate to be formed of the 5 by 3 array of
cells which is then inserted into the plastic tile
surround from behind. The surround (a) and the PV
laminate (b) for a PV tile (c) based around a Redland Figure 3: PV cells integrated into a tile

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Stonewold II tile (2x cover width) is shown in Fig. 3.
The plastic laminate which holds the PV cells produces a robust surround which protects the cells from
damage. This contrasts with traditional glass sandwich type PV modules which are very fragile and
need careful handling.
Installation
Approximately 15% of the energy adsorbed by a monocrystalline PV cell is converted to electrical
energy. The majority of the absorbed sunlight is dissipated as heat. The efficient dissipation of this heat
represents a particular problem for roof integration. The sarking felt which sits on the rafters is
generally a bitumen based material which softens at about 80 oC. Studies have shown that in a
standard roof covered with ordinary PV modules temperatures can approach this level [2, 3]. The
efficiency of PV cells is temperature dependant, with a rise in temperature having a corresponding
reduction in cell efficiency. Therefore, control of the temperature of a PV roof is necessary in order to
maintain both satisfactory array performance and sarking felt integrity. To provide sufficient ventilation it
is necessary to increase the distance between the sarking felt and the rear of the PV tiles. A spacer
must be placed behind the battens to increase the tile - felt gap to approximately 10 cm. A convection
process can therefore, be established allowing air to enter through holes in the soffit and exit at the top
of the roof. An added benefit of PV tiles in this respect is that standard tiles specifically designed for
ventilation purposes can be used.
A typical photovoltaic household application is for a 3 kW system. Currently, standard PV modules cost
about $4.5/W (£3/W) although prices are continually falling. The cost of the photovoltaic material, for a
standard, module based system is nearly £10,000. In addition to this, metering, installation, cabling and
an inverter to convert the produced DC power to domestic rating AC is required. The total cost of a
system is approximately £15,000-20,000.
The payback times for PV systems in pure economic terms are very long at present. To maximise the
benefit of a PV system it is important that the user makes use of the 25 year working life. Modern life
however, is increasingly transient in nature, with the "job for life" expectations of previous generations
no longer holding true. Short term employment contracts lead to the frequent need to move in order to
find work. Traditional PV installations cannot easily be moved to another house as they are generally a
custom design for a specific building. It is highly unlikely that on the sale of a house anywhere near the
£15-20 K investment would be recovered. A PV system based on PV roof tiles offers a distinct
advantage in this respect. The tiles can simply be removed from the roof and replaced with the
analogous standard roof tiles with relative ease. The PV tiles can then be transported to the new house
for re-use. The universal application of PV tiles will also generate a market for "secondhand" tiles.
Conclusions
Continual improvements in cell efficiencies coupled with reductions in production costs through new
processes and economies of scale will enable PV to become an economic reality for urban power
generation in the near future. Photovoltaic roof tiles have the potential to bring PV to the largest U.K.
market, namely the home. A design such as that described in this paper, will enable PV to be integrated
in an aesthetically pleasing manner. This is crucial to the success of the product in a market where
consumers demand “green” energy alternatives but are reluctant to compromise their surrounding
environment as a result. In addition, unlike other forms of PV installation, specialist contractors will not
be needed for PV roof tile installations. A skilled roofing “trade” is readily available and would need no
extra training to commission PV tile roofs In essence, the PV roof tile represents the combination of a
mature tile industry, with state of the art plastic and photovoltaic technologies. In this way, a new
approach to the integration of photovoltaics in buildings can be realised.

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References
[1] Bahaj A.S. & Ward S.C.,
The SOLATILE : A fully adjustable and Integrated Photovoltaic Roof
tile
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[2] Yang H.X., Marshall R.H. & Brinkworth B.J.,
An experimental study of the Thermal Regulation
of a PV-Clad Building Roof
, Proceedings of 12th European Photovoltaic Solar Energy
Conference, pp 1115-1118, 1994.
[3] Okuda N., Yagiura T., Morizane M., Ohnishi M. & Nakano S.,
A new type of Photovoltaic
Shingle
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pp 1008-1011, 1994.